This application claims the benefit of International Application No. PCT/JP99/02048, which has the international filing date of Apr. 19, 1999, and which was not published under PCT Article 21(2) in English.
1. Technical Field
The present invention relates to a solid electrolyte type fuel cell (hereinafter sometimes referred to as SOFC) having high power generating performance and durability.
2. Background Art
A solid electrolyte type fuel cell comprises an air electrode, a solid electrolyte film, a fuel electrode and an interconnector. In the following, the respective prior art techniques are explained.
First, with regard to an air electrode, prior art is explained by referring to an air electrode or an air electrode supporting tube of a solid electrolyte type fuel cell of a cylindrical cell type as an example. Solid electrolyte type fuel cells are disclosed in Japanese Patent publication number Kokoku Hei:1-59705, etc. The solid electrolyte type fuel cell has a cylindrical cell constituted by a porous supporting tube-an air electrode-a solid electrolyte-a fuel cell-an interconnector. When oxygen (air) is flown to the air electrode side and a fuel gas (H2, CO, etc.) is flown to the fuel electrode side, O2xe2x88x92 is moved in the cell to cause a chemical burning whereby potential difference between the air electrode and the fuel electrode occurs to cause power generation. There is a system in which the air electrode also has a function of a supporting tube (an air electrode supporting tube).
As a material for an air electrode of a solid electrolyte type fuel cell, a perovskite type oxide ceramics has been proposed such as LaMnO3 in Japanese Patent publication number Kokoku Hei:1-59705, and La1xe2x88x92xSrxMnO3 in Japanese Patent publication number Kokai Hei:2-288159. Also, in Proc. of the 3rd Int. Symp. on SOFC, 1993, La0.09Sr0.10MnO3 has been introduced as an air electrode.
A size of an air electrode supporting tube is generally an outer diameter of 10 to 20 mm, a thickness of 1 to 2 mm and a length of 1 to 2 m. For producing such a long ceramics formed product, an extrusion forming method has generally been used.
For producing such a long ceramics sintered body, a bend to the lateral direction becomes a problem. As a method of decreasing a bend of a long sintered body, a hanging sintering is carried out. The hanging sintering is to carry out the sintering in the state that a material to be sintered is hanged to the longitudinal direction. A tensile stress by weight of the material to be sintered itself is affected to the sintered material so that a correcting force is applied to the sintered body whereby a sintered body with a little bend can be obtained.
In Japanese Patent publication number Kokoku Hei: 6-10113, with regard to a production process of a long sintered body, there is disclosed a technique in which a ceramics long body is subjected to lateral sintering at a temperature not less than a shrinkage starting temperature of said ceramics material, and then, subjecting to hanging sintering at a temperature not less than the lateral sintering temperature. According to said publication, it is described that a rod of ZrO2 with a length of 1.1 m and a diameter of 20 mm is sintere data lateral sintering temperature between 1300 to 1450xc2x0 C. and a hanging sintering temperature of 1450xc2x0 C. so that a good result can be obtained.
In La0.09Sr0.10MnO3 which is representative as an air electrode composition, there are cases where gas permeability is insufficient so that high power generating performance cannot be obtained. On the contrary, when gas permeability is heightened to improve power generating performance, strength is lowered so that a cell is broken during power generation or a cell is broken during preparation thereof
As a preparation method of a perovskite type oxide, a method of repeating pulverizationxe2x80x94pressurizingxe2x80x94heat treatment is disclosed in Japanese Patent publication number Kokai Hei:7-6769. As a pulverizing method of ceramics powder, an impact type pulverizer, a stream type pulverizer, aba mill, etc. has been used, and the impact type pulverizer has been used in many cases in the point of easiness in operation.
In the case of the present method, a metal material containing Fe as a main component has been used as a material for a pulverization blade or a pulverization room. Thus, when the above-mentioned pulverization is repeated by using, for example, stainless as a material of the pulverization blade or the pulverization room, 0.5 to 1.0 w % or so of Fe component is migrated in the synthesized lanthanum manganite powder. Experiments were carried out with regard to a Fe content in the lanthanum manganite and power generating performance of a cell, and as a result, it was found that the Fe component in the lanthanum manganite markedly affects to the power generating performance of the cell and the Fe component with 0.5 wt % or more markedly lowers the power generating performance of the cell.
In a material other than the long sintered body comprising a material or having a size specifically mentioned as an object in the above-mentioned Japanese Patent publication number Kokoku Hei:6-10113, if a temperature of a lateral sintering before a hanging sintering is lowered, sintering at the lateral sintering does not proceed sufficiently so that high temperature strength is insufficient whereby there is a fear of dropping the sintered body by breaking the sintered body due to itself weight in many cases in a subsequent hanging sintering. For example, a shrinkage starting temperature of the lanthanum manganite is 1000xc2x0 C. When a sintered body subjected to lateral sintering within the range of 1000 to 1350xc2x0 C. is to be subjected to hanging sintering at a temperature exceeding 1350xc2x0 C., there is an extremely high risk of dropping the material. In a solid electrolyte type fuel cell, it is preferred that a roundness of an air electrode supporting tube is 97% or more to decrease contact resistance when the cell is stacked. When the lateral sintering temperature is raised, the roundness is lowered so that it is difficult to use as an air electrode supporting tube for a solid electrolyte type fuel cell.
Prior art of a solid electrolytic film is explained. In SOFC, solid electrolyte thin films having permeability of oxygen ions (O2xe2x88x92) and impermeability of gas are required. These solid electrolyte thin films (ZrO2 base, CeO2 base, and such) are required to be thin and tight to achieve these characteristics.
In addition, they are required to be economically formed into large sized thin films. For the cell of SOFC for power generation, in general, a solid electrolytic film having a thickness of 30 to 2000 xcexcm is formed on a porous substrate having a thickness of 0.3 to 5.0 mm. Further, over the solid electrolyte thin film, a fuel electrode (made of Ni base cermets, and such materials) is formed.
For SOFC cell, in order to obtain solid electrolyte thin films which are thin and tight, and at the same time, low in production cost and excellent in mass productivity as a goal, the following have been proposed in the past.
Production Method by CVD.EVD (Chemical Electrical vapor deposition) (Japanese Patent Publication Number Kokai Sho: 61-91880):
This production method is characterized in that the first electrode is adhered onto a porous support member, an intermediate layer substance with electrical conductivity and oxygen permeability is adhered onto the first electrode to protect the first electrode from a high temperature vapor of metal halides, and the intermediate layer substance is contacted with a high temperature vapor of metal halide to form a solid electrolyte composed of metal oxides on the whole surface of the intermediate layer.
Production Method by Plasma Spray Coatings (Japanese Patent Publication Number Kokai Sho: 61-198570):
This production method is characterized in that solid electrolyte starting materials comprising zirconium oxides and metal oxides of rare earth elements and the like are formed into a solid solution. Then, the starting material in the form of a solid solution are crushed, and the grain size of the obtained solid electrolyte powder is regulated, and then the powder is coated as an electrolyte film on a fuel cell substrate by plasma spray coating. According to an example described in the published specification, a solid electrolyte thin film having a thickness of 200 xcexcm and terminal voltage of 790 mV is obtained by using plasma spray powder having a grain size of 2 xcexcm or less.
Production Method by Slurry Coating (Japanese Patent Publication Number Kokai Hei:1-93065)
This production process is characterized in that either one of the air electrode layer or the fuel electrode layer is formed into a cylindrical shape. A powder slurry of each material for forming the electrolyte and the other electrode layer is coated on the surface of the cylinder one by one and dried, and then the cylinder is sintered. According to an example of the published specification, an YSZ thin film having a thickness of 150 xcexcm is obtained.
Production Method by Plasma Spray Coating and Filler Slurry Coating (Japanese Patent Publication Number Kokai Hei:2-220361)
This production method is characterized in that a filler material containing 40 wt % or more as solids concentration of yttrium stabilized zirconia is coated on voids of a solid electrolyte layer formed on the substrate cylinder by spraying, and then the tube is dried and sintered. According to an example described in the published specifications a slurry containing YSZ powders with a grain size of 0.05 to 2.5 xcexcm is coated (brush hand coating) on an air plasma spray coated film having a thickness of 100 xcexcm, and then the film is dried and sintered. Finally, a solid electrolyte thin film with very low gas permeability is obtained.
The aforesaid conventionally proposed techniques involve the following problems.
CVD Method, EVD Method:
These methods are appropriate for forming tight thin films. However, they require expensive equipment since film forming should be carried out under a special atmosphere and a physical condition isolated from the air. For large sized parts, large sized equipment is naturally required to accommodate the parts. Accordingly, film coating onto large parts is difficult as well as low in productivity and high in cost. Also, a corrosive starting gas is used so that there is a high risk that the substrate is corroded.
Plasma Spray Coating Method:
The films obtained by this method is fundamentally porous. Therefore, the film should be made relatively thick to eliminate gas permeability. Thus, high performance cell cannot be obtained. Also, its mass productivity is low.
Slurry Coating Method:
This method is an economical method since film formation is carried out in the air and expensive equipment is not necessary. This method, however, has been found to have problems in tightness and thickness of the film. Practically, and the solid electrolyte thin film disclosed as an example in the Japanese Patent publication number Kokai Hei:2-220361 has a thickness of 200 xcexcm, which is considerably thicker than the target thickness of 10 to 50 xcexcm for this kind of films. Also, sintering crack of the film was likely caused so that a plural times of sintering were required for obtaining a tight material while filling the crack. To solve such a problem and to increase sintering property of the film material, raising of the sintering temperature or finely pulverizing of slurry powder has been investigated. However, with regard to the former, a reaction between the substrate and the solid electrolyte becomes a problem, and in the latter, there is a difficulty in mass production of fine powder having a grain size of 0.1 xcexcm or less.
Plasma Spray Coating and Slurry Filling Method:
This method requires two-step work, and moreover the film thickness tends to be thicker.
Prior art technique of a fuel electrode is to be explained.
As a material for the fuel electrode of SOFC, a sintered layer of a composite powder in which NiO and Y2O3-stabilized ZrO2 (YSZ) are mixed and composited (Japanese Patent publication number Kokai Sho:61-153280, Japanese Patent publication number Kokai Sho:61-198570, etc.) has been mainly used. Incidentally, NiO in the sintered layer is reduced to Ni during operation of SOFC and said layer becomes a cermet film of Ni/YSZ.
As a production method of starting powder for Ni/YSZ cermet, there has been generally employed a method (a solid mixing method) in which NiO powder and YSZ powder are mixed both in solid states, and then, the temperature is elevated (calcination) to slightly sinter the material whereby it is composited. As a mixing method, there has been known a method of using a ball mill or a method by mechano-chemical mechanical mixing.
The nickel base/zirconium base powder which has been obtained by the above-mentioned prior art techniques had a structure in which Ni grains or NiO grains and YSZ grains as electrolyte material had been simply dispersed after all. In the state in which Ni grains and YSZ grains having sole grain size are dispersed, under operating conditions of SOFC (1000xc2x0 C., reduction), fine cracks occur on the surface of the fuel electrode due to reduction to break off a conductive pass which causes lowering in conductivity whereby lowering in output occurs.
Prior art technique of an interconnector is to be explained. For the interconnector of SOFC, the following characteristics are required.
To have a high electric conductivity. A role of the interconnector is to have an electrical connection between unit cells of SOFC so that it is the most fundamental required matter.
If the electric conductivity is low, self consumption of a power in the interconnector becomes large so that power generation performance of the cell is lowered. The electric conductivity is required to be 10 Sxc2x7cmxe2x88x921 or more (more preferably 40 Sxc2x7cmxe2x88x921 or more) at the film-formed state.
To have a low gas permeability. At the both surfaces of the interconnector, a fuel gas (H2, CO, etc.) and an oxidizing agent (air, etc.) flow, but if they are mixed through the interconnector, power generation performance of the cell is lowered. The gas permeability is required to be 0.01 (mxc2x7hrxe2x88x921xc2x7atmxe2x88x921) or less (more preferably 0.0001 (mxc2x7hrxe2x88x921xc2x7atmxe2x88x921) or less).
To have durability to both of oxidation and reduction.
To have a thermal expansion coefficient close to that of constitutional materials of the other cells such as YSZ (yttria stabilized zirconia).
To have low reactivity to an air electrode material such as LaSrMnO3, LaCaMnO3, etc., and YSZ.
To have a form ability into thin film. Through the interconnector, a current flows to the thickness direction so that a thin material has a less resistance. It is required to have a film thickness of 200 xcexcm or less.
Heretofore, as can be seen in Japanese Patent publication number Kokai Sho:61-153280, as a production process of an interconnector film of a cylindrical cell type-solid electrolyte type fuel cell, when those shown in said publication and carried out by the CVD-EVD method are described in detail, they are as follows:
In this method, a first reacting agent containing an oxygen source transmits through fine poreportion in a substrate material, and reacts with a halogenated metal gas at the other side of the substrate to form a film of metal oxide on the substrate. Accompanying with growth of the metal oxide which is a reaction product on the substrate, according to chemical vapor deposition (CVD), the reaction product closely shut out a plural number of fine pore portions in the substrate. Transfer of oxygen through an oxide layer in the course of growth from the oxygensource occurs so that the coated film is grown by the electrochemical vapor deposition (EVD).
Heretofore, as a collecting method of an interconnector comprising lanthanum chromite, electricity generation tests were carried out by directly connecting the lanthanum chromite and the collecting member.
In Japanese Patent publication number Kokai Hei:4-282566, as a structure of an interconnector, it is described that an antioxidative ceramics layer and a reduction-resistant metal layer are to be preferred.
In the conventional CVD.EVD method, it is suitable for forming a tight film having good adhesiveness. However, film formation should be carried out under a specific atmosphere and a physical condition isolated from the air so that an expensive device is required. For large sized parts, large sized equipment is naturally required to accommodate the parts. Accordingly, film coating onto large parts is difficult as well as low in productivity and high in cost. Also, in CVD.EVD, a composition of a material of the film to be formed is limited.
When electricity collection is carried out by contacting a lanthanum chromite membrane and a collecting member, contact resistance with the collecting member becomes large, and an output loss accompanied with joule heat loss is large whereby high output cannot be obtained.
In Japanese Patent publication number Kokai Hei: 4-282566, as a structure of an interconnector, it comprises an oxidation resistant ceramics layer and an reduction resistant metal layer. When the interconnector film having this structure is to be used in SOFC, sintering under oxidative atmosphere is difficult in order to form a metal layer so that preparation by the usual wet method is difficult and the production method limited to the CVD method or the plasma spray coatingmethod. It is possible to prepare the interconnector film according to these methods, but as mentioned above, productivity is low and a cost is high so that they are not practical methods. Also, preparation of a metal layer is possible even when a plating method is used, but this method requires a number of steps so that a cost is high whereby it is not practical.
A first object of the present invention is to provide an air electrode having high power generation performance and excellent in durability.
A second object of the present invention is to provide a formation of a tight solid electrolyte film excellent in economicity, mass productivity, easily capable of applying to a large surface area, low resistance and a uniform thin film within the range of 5 to 150 xcexcm.
A third object of the present invention is to provide a production of a nickel base/zirconium base complex powder as a fuel electrode material, which can inhibit aggregation of Ni in a long term operation, and a fuel electrode film.
A fourth object of the present invention is, in the preparation of an interconnector film on an air electrode substrate, to prepare a tight interconnector film by a wet method by providing a tight ceramics film (hereinafter referred to as a ceramics intermediate layer) with a certain extent between the air electrode substrate and the interconnector film, and further to provide a method for preparing an interconnector film which can decrease an output-loss accompanied with contact resistance with a collecting material at power generation and can attain high output of a cell by providing a nickel oxide film at a reductive atmosphere side of the interconnector film.
As the first aspect of the present invention, an air electrode for a solid electrolyte type fuel cell comprises an air electrode supporting tube also serving also as a supporting tube, and having radial crushing strength of 15 MPa or more and a gas permeation coefficient of 3.5 m2xc2x7hrxe2x88x921xc2x7atmxe2x88x921 or more. More preferably, it has the radial crushing strength of 20 MPa or more and the gas permeation coefficient of 3.5 m2xc2x7hrxe2x88x921xc2x7atmxe2x88x921 or more.
Further preferably, it has the radial crushing strength of 20 MPa or more and the gas permeation coefficient of 5.0 m2xc2x7hrxe2x88x921xc2x7atmxe2x88x921 or more.
The air electrode supporting tube having the above characteristics has a composition of (Ln1xe2x88x92xSrx)1xe2x88x92aMnO3, wherein 0.14xe2x89xa6xxe2x89xa60.26, 0 less than axe2x89xa60.03, Ln=at least one of La, Ce, Nd, Pr and Sm. Or else, it has a composition of (Ln1xe2x88x92xCax)1xe2x88x92aMnO3, wherein 0.20 less than x less than 0.35, 0 less than axe2x89xa60.03, Ln=at least one of La, Ce, Nd, Pr and Sm. More preferably, it has a composition of (Ln1xe2x88x92xSrx)1xe2x88x92aMnO3, wherein 0.16xe2x89xa6xxe2x89xa60.21, 0 less than axe2x89xa60.03, Ln=at least one of La, Ce, Nd, Pr and Sm. Or it has a composition of (Ln1xe2x88x92xCax)xe2x88x92aMnO3, wherein 0.25xe2x89xa6xxe2x89xa60.30, 0 less than axe2x89xa60.03, Ln=at least one of La, Ce, Nd, Pr and Sm.
As a production method, in the composition of (Ln1xe2x88x92xSrx)1xe2x88x92aMnO3, wherein 0.16xe2x89xa6xxe2x89xa60.21, 0 less than axe2x89xa60.03, Ln=at least one of La, Ce, Nd, Pr and Sm, or in the composition of (Ln1xe2x88x92xCax)1xe2x88x92aMnO3, wherein 0.25xe2x89xa6xxe2x89xa60.30, 0 less than axe2x89xa60.03, Ln=at least one of La, Ce, Nd, Pr and Sm, the process comprises the steps of adding, to the above-mentioned ceramics powder (coarse powder) having a grain size distribution within the range of 10 to 150 xcexcm, or within the range of 10 to 200 xcexcm, ceramics powder (fine powder) having a finer grain size distribution than the above-mentioned coarse powder to prepare mixed powder of coarse powder and fine powder, forming and sintering the mixed powder of coarse powder and fine powder.
Also, lanthanum manganite powder to be used as a starting material has a relative density of 97% or more. More preferably, lanthanum manganite powder has a relative density of 98% or more.
In the above-mentioned air electrode supporting tube, it comprises an Fe content of 0.01 wt % or more to 0.5 wt % or less. More preferably, it comprises an Fe content of 0.01 wt % or more to 0.4 wt % or less.
In a sintering method of the above-mentioned air electrode support, it contains a step of sintering a long sintering body in the state in which it is substantially free from tensile stress to the longitudinal direction (lateral sintering step) and a step of sintering the long sintering body in the state in which it is hanged to the longitudinal direction after lateral sintering (hanging sintering step), wherein a sintering temperature of the lateral sintering step is set at 1400xc2x0 C. or higher and a sintering temperature at the hanging sintering step is made higher than that of the lateral sintering step.
That is, after sintering the material at a sufficiently high sintering temperature in the lateral sintering step, the hanging sintering is carried out at a temperature higher than that of the lateral sintering temperature. The state of the sintering body during the lateral sintering is basically placed horizontally, but when a tensile stress which causes a problem is not applied to the material, it is not limited to the above. In the air electrode support for a solid electrolyte type fuel cell of the present invention, in order to ensure high cell output characteristics of 0.2 W/cm2 or more, it is preferable that a gas permeation coefficient is 3.5 m2xc2x7hrxe2x88x921xc2x7atmxe2x88x921 or more. In order to ensure high cell output characteristics of 0.3 W/cm2 or more, it is preferable that a gas permeation coefficient is 5.0 m2xc2x7hrxe2x88x921xc2x7atmxe2x88x921 or more.
Also, to improve a preparation yield in a cell preparation step and to prevent from breakage of a cell during power generation, radial crushing strength of an air electrode support is preferably, 15 MPa or more, more preferably 20 MPa or more.
In the air electrode support for a solid electrolyte type fuel cell of the present invention; it preferably has a composition of (Ln1xe2x88x92xSrx)1xe2x88x92aMnO3, wherein 0.14xe2x89xa6xxe2x89xa60.26, 0 less than axe2x89xa60.03, Ln=at least one of La, Ce, Nd, Pr and Sm. Or else, it preferably has a composition of (Ln1xe2x88x92xCax)1xe2x88x92aMnO3, wherein 0.20xe2x89xa6xxe2x89xa60.35, 0 less than axe2x89xa60.03, Ln=at least one of La, Ce, Nd, Pr and Sm. This is because if a Sr doped amount is less than 0.14 or exceeds 0.26, or a Ca doped amount is less than 0.20 or exceeds 0.35, it is difficult to simultaneously ensure both of a gas permeation coefficient of 3.5 m2xc2x7hrxe2x88x921xc2x7atmxe2x88x921 or more and radial crushing strength of 15 MPa or more. If a exceeds 0.03, it is difficult to ensure a gas permeation coefficient of 3.5 m2xc2x7hrxe2x88x921xc2x7atmxe2x88x921 or more.
Also, it is more preferable to have a composition of (Ln1xe2x88x92xSrx)1xe2x88x92aMnO3, wherein 0.16xe2x89xa6xxe2x89xa60.21, 0 less than axe2x89xa60.03, Ln=at least one of La, Ce, Nd, Pr and Sm or a composition of (Ln1xe2x88x92xCax)1xe2x88x92aMnO3, wherein 0.25xe2x89xa6x less than 0.30, 0xe2x89xa6axe2x89xa60.03, Ln=at least one of La, Ce, Nd, Pr and Sm. When the composition is within the above range, it becomes possible to simultaneously ensure both of a gas permeation coefficient of 3.5 m2xc2x7hrxe2x88x921xc2x7atmxe2x88x921 or more and radial crushing strength of 20 MPa or more. At this composition, it is preferred to contain the steps of adding to coarse powder having a grain size distribution within 10 to 150 xcexcm, fine powder having a grain size distribution-within 0.1 to 5.0 xcexcm in an amount of 0.5 to 40 parts by weight to 100 parts by weight of the coarse powder and the fine powder in total to prepare a mixed powder of the coarse powder and the fine powder, and forming and sintering the mixed powder of the coarse powder and the fine powder. When the starting powder is within the grain size distribution, it is possible to simultaneously ensure both of a gas permeation coefficient of 3.5 m2xc2x7hrxe2x88x921xc2x7atmxe2x88x921 or more and radial crushing strength of 20 MPa or more.
Also, at this composition, it is preferred to contain the steps of adding fine powder having a grain size distribution within 0.1 to 5.0 xcexcm to coarse powder having a grain size distribution within 10 to 200 xcexcm in an amount of 0.5 to 40 parts by weight based on 100 parts by weight the coarse powder and the fine powder in total to prepare a mixed powder of the coarse powder and the fine powder, and forming and sintering the mixed powder of the coarse powder and the fine powder. When the starting powder is within the grain size distribution, it is possible to simultaneously ensure both of a gas permeation coefficient of 5.0 m2xc2x7hrxe2x88x921xc2x7atmxe2x88x921 or more and radial crushing strength of 20 MPa or more.
Also, to ensure radial crushing strength of an air electrode supporting tube of 15 MPa or more, it is preferable to make a relative density of lanthanum manganite powder 97% or more, and to ensure radial crushing strength of 20 MPa or more, it is preferable to make a relative density of lanthanum manganite powder 98% or more. In the present invention, to increase the density of the powder, a method of repeating pulverization, pressuring and heat treatment is employed, but the method is not limited thereto. For example, it is also possible by raising the heat treatment temperature, increase in compressing pressure, progress of pulverization before pressurizing, etc.
In an air electrode supporting tube made of lanthanum manganite according to the present invention, when this is used as an air electrode support for a solid electrolytic type fuel cell, to ensure high cell output characteristics of 0.2 W/cm2 or more, a Fe content is preferably 0.5 wt % or less, and to ensure high cell output characteristics of 0.3 W/cm2 or more, a Fe content is preferably 0.4 wt % or less.
Also, to improve preparation yield at a cell preparation step and to prevent from breakage of the cell during power generation, a Fe content is preferably 0.01 wt % or more.
An air electrode supporting tube of a solid electrolyte type fuel cell is preferably bend of 1.0 mm or less and roundness of 97% or more. In the preparation method of a conductive ceramics tube made of lanthanum manganite of the present invention, since the bend of the ceramics tube is 1.0 mm or less and the roundness is 97% or more, it is preferred that a long sintering body is sintered under the state in which it is substantially free from tensile stress to the longitudinal direction and the long sintering body is sintered in the state in which it is hanged to the longitudinal direction after lateral sintering, and a sintering temperature of the lateral sintering step is set to 1400xc2x0 C. or higher and a sintering temperature at the hanging sintering step is made higher than that of the lateral sintering step. When the lateral sintering temperature is less than 1400xc2x0 C., a risk of dropping by self-weight is high at the time of sintering under hanging and production yield is poor, and when the hanging sintering temperature is less than the lateral sintering temperature, the roundness becomes less than 97%.
In the above-mentioned lateral sintering step, sintering is carried out by placing a sintering body in grooves of a setter having the grooves, and here, said grooves preferably have substantially the same sectional profile as the sectional profile of the sintered body. To prevent deformation during the lateral sintering as much as possible, the grooves of the above-mentioned setter serves also as preventing deformation.
Particularly when a material such as lanthanum manganite base or lanthanum chromite base is sintered, high purity alumina is preferable as a setter material. These materials have high reactivity with Si, so that a fire resistant material containing no Si is preferred as a setter material. At the present stage, high purity alumina is practically used as such a material. Here, preferred purity of the alumina is 99% or more.
Also, it is preferred to apply coating to the grooves of the setter with the same materials as those of the sintering body to be sintered. This is because change in the material of the sintered body caused by diffusion of the atom during sintering can be prevented as much as possible. As a method of coating, a slurry coating method can be employed. A thickness of the coating is preferably 0.05 to 2.0 mm.
As the second aspect of the present invention, the solid electrolyte film is a solid electrolyte material such as zirconia (ZrO2) type, ceria (CeO2) type material, and a specific surface area of the solid electrolyte powder is made 0.2 to 50 m2/g, or the heat treatment temperature of the solid electrolyte powder is set at 700 to 1600xc2x0 C.
It is to be included a step of preparing a solid electrolyte slurry with the above-mentioned solid electrolyte material and a step of film-forming the solid electrolyte slurry on an electrode substrate.
In the present invention, the characteristic feature resides in forming a tight solid electrolyte film on the substrate of an air electrode or a fuel electrode. In the sintering on the substrate, when a stress due to sintering shrinkage is larger than that the strength of, the substrate, the substrate is deformed or broken. Also, the stress due to sintering shrinkage is smaller than the same, crack occurs in the solid electrolyte film itself or sufficient tightness cannot be obtained. Accordingly, sintering of the solid electrolyte film itself shall be carried out without giving a large stress to the sintering substrate, so that a specific surface area of the solid electrolyte powder and the heat treatment temperature are controlled.
A solid electrolyte slurry is prepared by using the above-mentioned electrolyte material, and the solid electrolyte slurry is coated on an electrode substrate to form a film, whereby a tight and thin solid electrolyte film can be formed.
Also, it is preferred that before the step of forming a film of the solid electrolyte slurry on an air electrode substrate, a slurry containing mixed powder (LSCM/YSZ) of La(Sr,Ca)MnO perovskite base oxide (LSCM) and yttria doped zirconia (YSZ) is coated on the air electrode substrate to form a film.
Also, in the method of film-forming the above-mentioned solid electrolyte slurry on an air electrode substrate or on a fuel electrode substrate by dipping, film-formation can be carried out by separating the air electrode substrate or the fuel electrode substrate from the slurry at a speed of 5000 mm/sec or less.
The characteristic feature of the present invention resides in uniformly forming a tight solid electrolyte thin film by the slurry coating method.
Its characteristic feature is, in a solid electrolyte powder which at least one of Y2O3, Yb2O3, Dy2O3, Er2O3, Eu2O3, Gd2O3, Ho2O3, Lu2O3, Sm2O3 and Tm2O3 element is added to a solid electrolyte material such as zirconia base, ceria base, etc. which are stable as a solid electrolyte at high temperatures, to control sintering property, a specific surface area and a heat treatment temperature of the solid electrolyte powder are to be controlled.
Also, in the method of forming a film by dipping, by separating the air electrode substrate or the fuel electrode substrate from the slurry at a speed of 5000 mm/sec or less, whereby a slurry amount adsorbed or attached to the substrate is made constant so that film-formation with a uniform film thickness is carried out.
The reason why the specific surface area of the solid electrolyte powder in the present invention is 50 m2/g or less, or a heat treatment temperature is set at 700xc2x0 C. or higher is as follows. In a solid electrolyte powder having a specific surface area of larger than 50 m2/g, or in a solid electrolyte powder subjected to a heat treatment at a temperature lower than 700xc2x0 C., sintering properties are too high so that in sintering on an air electrode or a fuel electrode substrate, cracks sometimes occur in the solid electrolyte film or the substrate is deformed. Also, when a slurry is prepared by using the above-mentioned solid electrolyte powder, there is a problem that it is hardly mixed with a solvent uniformly, or the like.
The heat treatment temperature of the solid electrolyte powder according to the present invention is preferably 800xc2x0 C. or higher and 1500xc2x0 C. or lower. This is because, if the heat treatment temperature is too low, as mentioned above, sintering properties of the solid electrolyte powder are high so that a stress accompanied with sintering shrinkage of the solid electrolyte powder occurs at sintering on an air electrode or a fuel electrode substrate, and the substrateis damaged thereby. Also, if the heat treatment temperature is too high, it becomes difficult to grind the material until it has a suitable specific surface area.
When the heat treatment temperature is set a high temperature, e.g., it is set higher than 1600xc2x0 C., after the heat treatment, the material may be ground so that the specific surface area becomes 0.2 m2/g or more, preferably 2 m2/g or more within the range of 50 m2/g or less, preferably within the range of 40 m2/g or less.
The reason why a ball mill is used when a solid electrolyte slurry is prepared is in the points that it is easily ground to uniform and fine solid electrolyte powder in the specific surface area of 50 m2/g or less, and that a solvent and the solid electrolyte powder can be uniformly dispersed.
Also, when a solid electrolyte slurry is subjected to film-formation on an air electrode substrate, it is preferred to coat a slurry containing mixed powder (LSCM/YSZ) of La(Sr,Ca)MnO3 perovskite base oxide (LSCM) and yttria doped zirconia (YSZ) on an air electrode substrate to form a film before the step of coating the solid electrolyte slurry on the air electrode substrate to form a film.
By coating a slurry containing LSCM/YSZ mixed powder on the above-mentioned air electrode substrate to form a film, an interfacial conductivity between the air electrode and the solid electrolyte can be improved whereby a cell with high performances can be formed.
In the present invention, when a film is formed by dipping, the reason why the slurry is separated from the air electrode substrate or the fuel electrode substrate at a speed of 5000 mm/sec or less is as follows. If they are separated at a speed faster than 5000 mm/sec, difference in the film thickness of 2-folds or more occurs in the upper portion and the bottom portion of the film-formed substrate having a length of 1000 mm or so to the up and down direction.
The reason why the slurry is preferably separated from the air electrode substrate or the fuel electrode substrate at a speed of 500 mm/sec or less is as follows. If they are separated at a speed later than 500 mm/sec, difference in the film thickness can be controlled within 1.5-folds at the upper portion and the lower portion of the air electrode substrate or the fuel electrode substrate having a length of 1000 mm or so to the up and down direction.
The reason why the slurry is preferably separated from the air electrode substrate or the fuel electrode substrate at a speed of 50 mm/sec or less is as follows. If they are separated at a speed later than 50 mm/sec, difference in the film thickness can be controlled within 1.25-folds at the upper portion and the lower portion of the air electrode substrate or the fuel electrode substrate having a length of 1000 mm or so to the up and down direction.
Dipping of the present invention is preferably carried out with a plural number of times. It is also preferred that the upper portion of the air electrode substrate or the fuel electrode substrate comes to an upper side as the same times as the bottom portion of the air electrode substrate or the fuel electrode dose.
The reason is that a hanged amount of the slurry of the attached slurry can be made uniform at the up and down direction irrespective of the length of the air electrode substrate or the fuel electrode substrate whereby a film can be formed substantially without causing difference in film thickness to the up and down direction.
Dipping of the present invention preferably carried out a plural number of times, and the upper portion and the bottom portion of the air electrode substrate or the fuel electrode substrate are overturned alternately.
The reason is that a hanged amount of the slurry of the attached slurry can be made uniform at the up and down direction and an amount of the slurry adsorbed to the formed film can be made constant irrespective of the length of the air electrode substrate or the fuel electrode substrate whereby a film can be formed substantially without causing difference in film thickness to the up and down direction.
The reason why the slurry viscosity of the present invention is arranged to 1 to 500 cps, preferably 5 to 100 cps is as follows. The reason why the lower limited is restricted to 1 cps that, in the case of low viscosity, when a film is formed on a tight substrate, cissing occurs so that spots occurs in the film. Also, when a film is formed on a porous substrate, it is to prevent from penetrating the slurry into the substrate. The reason why the upper limit is restricted to 500 cps is to prevent from causing cracks during drying after film-formation.
As the dipping method of the present invention, in addition to the usual dipping method in which a substrate is dipped in a slurry in air, a method of dipping in a pressurized gas or in vacuum can be employed. In this case, a number of dipping times can be selected depending on the required film thickness and the slurry composition to be used.
The reason why a ratio of a solid electrolyte material occupied in an optional section of a solid electrolyte film is two thirds or more is that if it is less than two thirds, an inner resistance of the solid electrolyte film abruptly increases and electricity generation characteristics are markedly lowered. Also, voids at which no solid electrolyte material exists become a starting point of a cause of generating cracks.
As a film thickness of the solid electrolyte film, it is preferably 5 to 150xcexcm. If it is less than 5 xcexcm, a sufficiently tight film cannot be obtained while if it exceeds 150 xcexcm, an inner resistance is too large. It is preferably 100 xcexcm or less, more preferably 50 xcexcm or less.
The reason why the gas permeation coefficient of the solid electrolyte film is made 1xc3x9710xe2x88x929 m3xc2x7s kgxe2x88x921 or less is to operate SOFE with a high utilizing ratio of fuel. It is preferably 1xc3x9710xe2x88x9210 m3xc2x7sxc2x7kgxe2x88x921 or less.
The reason why the sintering temperature of the solid electrolyte film is set 1200xc2x0 C. to 1700xc2x0 C. is that if the sintering is carried out at a temperature lower than 1200xc2x0 C., sintering of the solid electrolyte powder does not proceed sufficiently so that a tight solide lectrolyte film cannot be obtained. Also, the sintering is carried out at the temperature higher than 1700xc2x0 C., porosity of the electrode substrate is lost.
In the solid electrolyte material made of a zirconia base, a ceria base, etc., the reason why at least one of Y2O3, Yb2O3, Dy2O3, Er2O3, Eu2O3, Gd2o3, Ho2O3, Lu2O, Sm2O3 and Tm2O3 element is contained in man amount of 3 to 20 mol is that ion conductivity is excellent in this range. It is preferably 8 to 12 mol %. Also, more preferably, in the zirconia base solid electrolyte material, Y2O3 is contained in an amount of 8 to 12 mol %.
The content of the solid electrolyte powder in the slurry of the present invention is preferably 10 parts to 50 parts to 100 parts of the slurry solution. The composition of the slurry solution of the slurry according to the present invention is not particularly limited. The slurry may contain a solvent, a binder, a dispersant, an anti-foaming agent, etc. However, it is preferable to contain, as a solvent, a hardly volatile solvent in an amount of 10 to 80 wt % based on the slurry solvent. An action of the hardly volatile solvent is to control viscosity change of the slurry at the time of preparing a slurry and during preservation, and to control occurrence of cracks caused by drying after film-formation (e.g., by dipping) by using this slurry. Here, the degree of hardly volatility is preferably, for example, 1 or less when the volatility of butyl acetate is set 100. For example, there may be mentioned xcex1 terpineol, etc.
In the slurry solution, a usual volatile solution may be contained other than the hardly volatile solvent. An action of the solvent contained in the solution is to improve dispersibility of the solid electrolyte powder and improve defoaming property. As an examples of such a solvent, ethyl alcohol is preferable. A preferable content thereof is 20 to 90 wt % of the slurry solution.
An action of the binder contained in the slurry solution is to improve coating property (adhesiveness) of the solid electrolyte powder to the substrate. An amount of the binder is preferably 0.1 to 10 parts to 100 parts of the solvent. The reason is that if it is a low concentration (less than 0.1 wt % or less), coating property is poor, while if it is a.high concentration (exceeding 10 wt %), dispersibility of the solid electrolyte powder becomes poor. Specific examples of the binder may be preferably mentioned ethyl cellulose.
An action of a dispersant contained in the slurry solution is to improve dispersibility of the solid electrolyte powder. An amount of the dispersant is preferably 0.1 to 4 parts to 100 parts of the solvent. The reason is that if it is a low concentration (less than 0.1 wt % or less), dispersibility is low, while if it is a high concentration (exceeding 4 wt %), modification of the slurry is likely caused. Specific examples of the dispersant may be preferably mentioned polyoxyethylene alkylphosphate.
An anti-foaming agent contained in the slurry solution has an action of defoaming bubbles in the slurry solution. An amount of the anti-foaming agent is preferably 0.1 to 4 parts to 100 parts of the solvent. The reason is that if it is a low concentration (less than 0.1 wt % or less), its effect cannot be expected, while if it is a high concentration (exceeding 4 wt %), modification of the slurry is likely caused. Specific examples of the anti-foaming may be preferably mentioned sorbitan sesquioleate.
As the mixing method of the respective agents and the solid electrolyte powder, a method of ball mill, etc. maybe employed.
The application method of the slurry to the substrate in. the production process of the present invention is not particularly limited. It may be a dipping method, spraymethod, brushing method, printing method, transfer method, etc. Of these, a dipping method is preferred. It is simple and easily, excellent in mass productivity and low cost. As the dipping method, in addition to the usual dipping method in which a substrate is dipped in a slurry in air, a method of dipping in a pressurized gas or in vacuum can be employed. In this case, a number of dipping times can be selected depending on the required film thickness and the slurry composition to be used.,
As the third aspect of the present invention, a fuel electrode material comprises a nickel base/zirconium base composite powder, and the nickel base/zirconium base composite powder is prepared by the step of synthesizing a composite powder comprising nickel and/or nickel oxide, and yttria doped zirconia (YSZ) and a step of subjecting to heat treatment after the above-mentioned powder is made a green compact. The heat treatment temperature is 1200 to 1600xc2x0 C.
Or else, it is a composite powder comprising nickel and/or nickel oxide and yttria doped zirconia (YSZ), and the nickel base/zirconium base composite powder is prepared by subjecting to calcination under air atmosphere or reductive atmosphere as a previous and by subjecting to heat treatment further after making the green compact. The calcination temperature and the heat treatment temperature are each 500 to 1200xc2x0 C. and 600 to 1200xc2x0 C. under both of the air atmosphere and the reductive atmosphere.
Or else, it is a composite powder comprising nickel and/or nickel oxide and YSZ, and the nickel base/zirconium base composite powder is prepared by synthesizing powder to which at least one of calcium, strontium and magnesium is doped to a zirconium element.
Or else, it is a composite powder comprising nickel and/or nickel oxide and YSZ, and the nickel type/zirconium type composite powder is prepared by synthesizing powder to which at least one of cobalt, aluminum, titanium and magnesium is doped to a nickel oxide.
Or else, it is a nickel base/zirconium base composite powder with an optional ratio of a nickel element, zirconium element and yttrium element, and the nickel base/zirconium base composite powder is prepared by mixing these composite powder having different grain sizes. A ratio of the grain sizes of the mixed powder is made xc2xd or less. Also, in a formulation of the mixed powder, smaller powder (fine powder) is made 5 wt % to 50 wt % to larger powder (coarse powder).
The above-mentioned powder can be obtained by synthesizing with a co-precipitation method by making starting materials water-soluble metal salts such as nitrates, sulfates, carbonates or chlorides, etc.
The powder is subjected to heat treatment at a temperature of 600 to 1600xc2x0 C. under air atmosphere or reductive atmosphere to produce a nickel base/zirconium base composite powder.
After subjecting to the heat treatment, graininess of the powder is regulated to 0.2 xcexcm to 50 xcexcm, and a slurry is prepared by using them and a film is formed by a slurry coating method.
The film material is sintered under air atmosphere at 1100xc2x0 C. to 1500xc2x0 C. or reductive atmosphere at 1100xc2x0 C. to 1400xc2x0 C. Incidentally, in the sintering under the reductive atmosphere, a side containing nickel and/or nickel oxide on the YSZ solid electrolyte film is made the reductive atmosphere, and a side containing LaMnO3 type air electrode of the YSZ solid electrolyte film is made an oxidative atmosphere, and the sintering is carried out.
As effects of the third aspect of the present invention, when nickel type/zirconium base composite powder is to be synthesized, pressure is applied to the powder to make a green compact, and after making NiO grains and YSZ grains in a tight state, a heat treatment at a high temperature is carried out to obtain sufficient adhesiveness between NiO and YSZ. Also, by effecting a reductive treatment of nickel type/zirconium type composite powder, Ni/YSZ composite powder may be prepared.
When a green compact of NiO/YSZ composite powder is used, at the time of operating SOFC (100xc2x0 C., reduction), NiO is reduced to Ni and a porosity of the fuel electrode film itself is increased. Due to increase in the porosity of the fuel electrode film, it is possible to make a fuel electrode film having a high gas permeability.
On the other hand, when a green compact of Ni/YSZ composite powder is used, particularly when reductive sintering is carried out, Ni has higher sintering property than NiO so that it is possible to form a strong and firm fuel electrode film at a lower temperature. Also, in an atmospheric sintering, Ni is oxidized to NiO during sintering, the same effect as the above-mentioned NiO/YSZ can be expected.
Moreover, as a previous step before making a green compact, by carrying out calcination under air atmosphere or reductive atmosphere, it is possible to more uniformly disperse the NiO grains or Ni grains and YSZ grains, and also adhesiveness between grains can be heightened.
By adding at least one of calcium, strontium and magnesium to the nickel base/zirconium base composite powder, sintering properties of the composite powder is improved so that conductivity is to be improved.
By adding at least one of cobalt, aluminum, titanium and magnesium to the nickel base/zirconium base composite powder, they are dispersed in Ni and/or NiO in the composite powder, and agglomeration of Ni can be prevented during a long term operation under high temperatures and reductive atmosphere.
In the synthesis of a fuel electrode powder, in the nickel base/zirconium base composite powder, these composite powder having different grain sizes are to be mixed. By mixing coarse powder to fine powder, sintering properties are improved and bonding forces between grains are increased whereby occurrence of fine cracks on the surface of the fuel electrode by reduction during a long term operation under high temperatures and reductive atmosphere can be controlled.
When the nickel base/zirconium base composite powder of the present invention is to be applied to a solid electrolyte type fuel cell, at an interface with the solid electrolyte, NiO and/or Ni is made 30 mol % to 50 mol %, and at an upper layer of the interface, NiO and/or Ni is made 50 mol % or more. The reason is because, at the interface with the solid electrolyte, adhesiveness with YSZ which is a solid electrolyte material is important and if NiO and/or Ni is 50 mol % or more, adhesiveness is lowered. Also, at the upper layer of the interface, high conductivity is required and if NiO and/or Ni is less than 50 mol %, conductivity is abruptly lowered.
A Y2O3 content of YSZ in the nickel base/zirconium base composite powder of the present invention is preferably 3 to mol %, more preferably 8 to 12 mol %. The reason is that an ion conductivity of YSZ to be used as the solid electrolyte is excellent in this range so that matching between the solid electrolyte and a fuel electrode is good.
The production process of the composite powder of the present invention is preferably a co-precipitation method. According to the co-precipitation method, a composite powder having a uniform structure and composition can be obtained and an interfacial conductivity between the electrode (the fuel electrode) and the solid electrolyte can be increased several. times as compared to a composite powder prepared by a solid mixing method.
As a pressure for forming a green compact of the nickel type/zirconium type composite powder, it is sufficient to make the pressure 100 kgf/cm2 or more. If it is 100 kgf/cm2 or less, sufficient green compact cannot be obtained.
In the nickel base/zirconium base composite powder which a green compact was made without effecting calcination, the heat treatment temperature is preferably 1200 to 1600xc2x0 C. If it is 1200xc2x0 C. or less, a tight powder which NiO and YSZ are sufficiently contacted to each other cannot be obtained. Also, if it is made 1600xc2x0 C. or more, sintering of YSZ in the nickel base/zirconium base composite powder is too progressed so that regulation of graininess by grinding and classification is difficult, and formation of a strong and firm fuel electrode film cannot be carried out at the time of film-formation and sintering which are the later steps.
When a green compact is prepared after subjecting to calcination, the calcination temperature is preferably 500xc2x0 C. to 1200xc2x0 C. in air atmosphere. If it is higher than 1200xc2x0 C., sintering of YSZ in the nickel base/zirconium base composite powder is excessively progressed in relation between the green compact preparation and the heat treatment temperature thereafter, and the powder becomes powder in which NiO grains and YSZ grains are not uniformly dispersed. The same can be applied to the preparation under the reductive atmosphere. Also, a temperature of subjecting a green compact of the nickel base/zirconium base composite powder to heat treatment under air atmosphere is preferably 600xc2x0 C. to 1200xc2x0 C. If it is lower than 600xc2x0 C., tight powder in which NiO and YSZ are sufficiently contacted cannot be obtained. Also, if it exceeds 1200xc2x0 C., sintering of YSZ in the nickel base/zirconium base composite powder is excessively progressed in relation with the calcination temperature, regulation of graininess by grinding and classification is difficult, and formation of a strong and firm fuel electrode film cannot be carried out at the time of film-formation and sintering which are the later steps. In the heat treatment under the reductive atmosphere, the same can be applied as under the air atmosphere.
In the nickel base/zirconium base composite powder which is not subjected to compression procedure, it is preferable to carry out the heat treatment at a temperature of 800xc2x0 C. to 1600xc2x0 C. If it is lower than 800xc2x0 C., sufficient sintering property cannot be obtained in the nickel base/zirconium base composite powder. Also, if it is made 1600xc2x0 C. or higher, sintering of YSZ in nickel base/zirconium base composite powder is excessively progressed, regulation of graininess by grinding and classification is difficult, and formation of a strong and firm fuel electrode film cannot be carried out at the time of film-formation and sintering which are the later steps.
Regulating of graininess of the slurry grains in the present invention can be carried out by classification, etc. after grinding. Also, the content of the ceramics grains in the slurry is preferably 10 parts to 50 parts to 100 parts of the slurry solution. A composition of the slurry solution of a slurry in the present invention is not specifically limited. The slurry may contain a solvent, a binder, a dispersant, an anti-foaming agent, etc. However, it is preferable to contain, as a solvent, a hardly volatile, solvent in an amount of 10 to 80 wt % to the slurry solvent. An action of the hardly volatile solvent is to control viscosity change of the slurry at the time of preparing a slurry and during preservation, and to control occurrence of cracks caused by drying after film-formation (e.g., by dipping) by using this slurry. Here, the degree of hardly volatility is preferably, for example, 1 or less when the volatility of butyl acetate is made 100. For example, there may be mentioned xcex1 terpineol, etc.
In the slurry solution, a usual volatile solution may be contained other than the hardly volatile solvent. An action of the solvent contained in the solution is to improve dispersibility of the solid electrolyte powder and improve defoaming property. As an examples of such a solvent, ethyl alcohol is preferable. A preferable content thereof is 20 to 90 wt % of the slurry solution.
An action of the binder contained in the slurry solution is to improve coating property (adhesiveness) of the powder to the substrate. An amount of the binder is preferably 0.1 to parts to 100 parts of the solvent. The reason is that if it is a low concentration (less than 0.1 wt % or less), coating property is poor, while if it is a high concentration. (exceeding 10 wt %), dispersibility of the powder becomes poor. Specific examples of the binder may be preferably mentioned ethyl cellulose.
An action of a dispersant contained in the slurry solution is to improve dispersibility of the powder. An amount of the dispersant is preferably 0.1 to 4 parts to 100 parts of the solvent. The reason is that if it is a low concentration (less than 0.1 wt % or less), dispersibility is low, while if it is a high concentration (exceeding 4 wt %), modification of the slurry is likely caused. Specific examples of the dispersant may be preferably mentioned polyoxyethylene alkylphosphate.
An anti-foaming agent contained in the slurry solution has an action of defoaming bubbles in the slurry solution. An amount of the anti-foaming agent is preferably 0.1 to 4 parts to 100 parts of the solvent. The reason is that if it is a low concentration (less than 0.1 wt % or less), its effect cannot be expected, while if it is a high concentration (exceeding 4 wt %), modification of the slurry is likely caused. Specific examples of the anti-foaming may be preferably mentioned sorbitan sesquioleate.
As the mixing method of the respective agents and the solid electrolyte powder, a usual method of ball mill, etc. may be employed.
The application method of the slurry to the substrate in the production process of the present invention is not particularly limited. It may be a dipping method, spray method, brushing method, etc. Of these, a dipping method is preferred. It is simple and easily, excellent in mass productivity and low cost. As the dipping method, in addition to the usual dipping method which a substrate is dipped in a slurry in air, a method of dipping in a pressurized gas or in vacuum can be employed. In this case, a number of dipping times can be selected depending on the required film thickness and the slurry composition to be used.
As a sintering temperature of the nickel base/zirconium base composite film is preferably 1100 to 1500xc2x0 C. under air atmosphere and 1100xc2x0 C. to 1400xc2x0 C. under reductive atmosphere. If it is lower than 1100xc2x0 C., it is close to the operating conditions of SOFC (1000xc2x0 C.) so that aggregation of Ni grains during operation of SOFC becomes large. Also, in sintering on the YSZ film which is a solid electrolyte, adhesiveness to YSZ cannot sufficiently be ensured.
Also, in air atmosphere, if it is higher than 1500xc2x0 C., it may become a cause of sintering crack of the electrolyte film, etc., and under the reductive atmosphere, if it is higher than 1400xc2x0 C., a melting point of Ni is at around 1450xc2x0 C. so that Ni is melted whereby it is difficult to ensure porous structure.
A temperature raising speed at the time of sihtering is preferably 300xc2x0 C./hr or less. If it is faster than 300xc2x0 C./hr, before NiO is sufficiently reduced to Ni during temperature raising, sintering of NiO proceeds. If sintering of NiO proceeds before NiO is reduced to Ni, a stress due to volume shrinkage occurs to the sintering film itself of the fuel electrode so that it becomes a cause of generating cracks.
As the fourth aspect of the present invention, when an interconnector film of a tight ceramics film (hereinafter also referred to as a tight ceramics film) is formed on an air electrode (hereinafter also referred to as aporous substrate), a ceramics intermediate layer is formed between the air electrode and the interconnector film so that it is possible to form a tight film of a sintering-resistant material such as lanthanum chromite, etc., by the wet method.
In the formation of an interconnector film of SOFC, by forming a lanthanum chromite film at the oxidation atmosphere side and forming a nickel oxide film at the reductive atmosphere side and then sintering, it is possible to form an interconnector film which can reduce output loss accompanied by contact resistance with a collecting material at the time of power generation, and realize high output of a cell by an inexpensive wet method.
In the present invention, it is said to preferably form a ceramics intermediate layer having a gas permeation flux Qxe2x89xa650 (mxc2x7hr1xc2x7atmxe2x88x921) between a porous substrate and a tight ceramics film. The reason is that if Q greater than 50 (mxc2x7hrxe2x88x921xc2x7atmxe2x88x921), gas sealing property of the tight ceramics film to be formed thereon becomes worse, particularly when a liquid phase sintering is carried out as an alkaline earth metal doped lanthanum chromite, if the ground is porous, diffusion of the liquid phase component occurs whereby it is difficult to become tight. From this viewpoint, the ceramics intermediate layer preferably has a gas permeability as little as possible.
A gas permeation flux Q2 of the tight ceramics film herein mentioned is preferably a gas permeation flux Q2xe2x89xa60.01 (mxc2x7hrxe2x88x921xc2x7atmxe2x88x921) measured between the porous substrate and the tight ceramics film. The reason is that when the tight sintered film is used as an interconnector film of SOFC, if the gas permeation flux Q2 greater than 0.01, an output of SOFC may be reduced.
In the present invention, it is preferred to subject to a surface roughening treatment after formation of the ceramics intermediate layer. The reason is that if a film-formation is carried out on a smooth ceramics substrate, adhesiveness to the film is poor and peeling of the film after sintering may be caused.
In the present invention, when a thickness of the ceramics intermediate layer which is tight in a certain degree before subjecting to surface roughening treatment is made t1 (xcexcm) and a film thickness after the surface roughening treatment is made t2 (xcexcm), it is made 0.01xe2x89xa6(t1xe2x88x92t2)/t1xe2x89xa60.2. If (t1xe2x88x92t2)/t1 less than 0.01, unevenness of the surface is too small so that adhesiveness of the film is poor and peeling of the film occurs. On the other hand, if (t1xe2x88x92t2)/t1 greater than 0.2, a function (Qxe2x89xa650 (mxc2x7hrxe2x88x921xc2x7atmxe2x88x921), etc.) of the ceramics intermediate layer may be impaired.
A method of the surface roughening treatment of the present invention is not particularly limited. There are a method of treating with a sand paper, a method of spraying an abrasive on the surface (blast abrasion) by a spray, a method of attacking the surface by using chemicals such as an acid, an alkali, etc., and the like. Of these, the method of blast abrasion is preferred since operating step can be finished within a short time.
The abrasive to be used in the surface roughening treatment of the present invention is not particularly limited. There may be used silicon carbide, boron carbide, alumina, diamond, zirconia, etc. Of these, alumina and zirconia are preferable.
In the formation of a tight ceramics film of the present invention, it is preferred that a film is formed by the film forming method containing the step of dipping the material in the state of applying a differential pressure between the film forming surface and the surface of the opposite side (an opposite film forming surface) and a step of drying, and then, sintering. The reason is that in the film preparation comprising only the step of dipping the material with a relatively low differential pressure of less than 1 atm and drying, since a filling degree of powder at the film-formed portion is low, a tight film can hardly be obtained after sintering. Also, if when a high differential pressure exceeding 20 atm is applied, unevenness in powder distribution attached to the film forming portion may be caused, and when it becomes this state, there is a fear of causing peeling of the film or sintering crack after sintering. From this viewpoint, the differential pressure is preferably 1 atmxe2x89xa6xcex94Pxe2x89xa620 atm.
In the formation of a tight ceramics film of the present invention, it is preferred to form a film from a slurry containing powder having low sintering property and a slurry containing powder having high sintering property and sinter the film. The reason is that in a film-formed by using powder having low sintering property only, it is difficult to obtain a film having a high tightness, while in a film-formed by using powder having high sintering property only, peeling of the film or sintering crack after sintering may be caused.
The composition of the powder having low sintering property and the composition of the powder having high sintering property in the preparation of a tight ceramics film of the present invention are not particularly limited. They may be the same composition or different compositions, and further may be different materials.
For example, in the case of the same composition, Sr doped lanthanum chromite, Ca doped lanthanum manganite, etc., may be considered, while in the case of the different compositions, a combination of La0.8Ca0.2CrO3 and La0.7Ca0.3CrO3 or a combination of La0.8Ca0.2CrO3 and (La0.8Ca0.2)(Cr0.9Cu0.1)O3, and the like, can be considered. In the case of the different materials, a combination of Ca doped lanthanum chromite and calcium titanate, Ca doped lanthanum chromite and Ca doped lanthanum manganite, and the like, can be considered.
When the powder having low sintering property and the powder having high sintering property in the tight ceramics film of the present invention are the same composition, powder having high calcination temperature is made powder having low sintering property, and when they are different compositions or different materials, that having a low shrinkage ratio is made powder having low sintering property in the case of forming a press body and sintering the same temperature.
A calcination temperature of the powder having low sintering property to be used in the formation of the tight ceramics film according to the present invention is preferably 1000 to 1400xc2x0 C. The reason is that if the powder having a calcination temperature of less than 1000xc2x0 C. is used, sintering property is too high so that peeling of the film or sintering crack may be caused, while if it exceeds 1400xc2x0 C., sintering property is low so that a film having high tightness can hardly be obtained.
An average grain size of the powder having low sintering property to be used in the formation of the tight ceramics film according to the present invention is preferably 0.1 to 2 xcexcm. The reason is that if the grain is larger than 2 xcexcm, a tight film can hardly be obtained, while if it is less than 0.1 xcexcm, sintering property is improved so that there is a fear of causing peeling of a film or sintering crack.
A calcination temperature of the powder having high sintering property to be used in the formation of the tight ceramics film according to the present invention is preferably 800 to 1100xc2x0 C. The reason is that if it is less than 800xc2x0 C., sintering property is improved excessively so that it causes peeling of the film or sintering crack, while if it exceeds 1100xc2x0 C., sintering property is low so that a tight film can hardly be obtained.
An average grain size of the powder having high sintering property to be used in the formation of the tight ceramics film according to the present invention is preferably 0.1 to 1 xcexcm. The reason is that if the grain is larger than 1 xcexcm, a tight film can hardly be obtained, while if it is less than 0.1 xcexcm, sintering property is improved so that there is a fear of causing peeling of a film or sintering crack.
In the ceramics intermediate layer according to the present invention, formation of a film has been carried out by using two kinds of slurries, one of which is a coarse powder slurry containing a film substance having a relatively large grain size and the other is a fine powder slurry containing a film substance having a relatively small grain size, to form a film by sintering. This is because if the coarse slurry is used only, sintering property is low so that formation of a film having a tightness in a certain degree of Qxe2x89xa650 (mxc2x7hrxe2x88x921xc2x7atmxe2x88x921) is difficult, while if the fine slurry is used only, sintering property is too high so that shrinkage at sintering is too high whereby sintering crack or peeling of the film is likely caused.
In the formation of the ceramics intermediate layer according to the present invention, when the coarse powder composition is (La1xe2x88x92X1MX1)Y1MnO3, and the fine powder composition is (La1xe2x88x92X2MX2)Y2MnO3, then, it is preferred that they are within the range of 0 less than X1xe2x89xa6X2xe2x89xa70.4, 0.9xe2x89xa6Y1 less than 1, and 0.9xe2x89xa6Y2xe2x89xa61.
The reasons are that if X1=0 or X2=0, sintering property of the material itself is low so that formation of a film having a tightness in a certain degree of Qxe2x89xa650 (mxc2x7hrxe2x88x921xc2x7atmxe2x88x921) is difficult, while if X1 or X2 greater than 0.4, sintering property is too high so that sintering crack or peeling of the film may be caused.
If Y1 or Y2 less than 0.9, the manganese component is easily liberated or diffused and there is a problem in durability of the material, while if Y1 or Y2 greater than 1, sintering property is markedly lowered so that it is difficult to obtain a tight film.
In the ceramics intermediate layer according to the present invention, when the calcination temperature of the coarse powder is T1 and the calcination temperature of the fine powder is T2, it is preferred to be T1xe2x89xa7T2. The reason is that if the calcination temperature of the fine powder is higher than the other, a role of the fine powder as a sintering aid is reduced and it is difficult to form a film having a tightness in a certain degree of Qxe2x89xa650 (mxc2x7hrxe2x88x921xc2x7atmxe2x88x921).
In the ceramics intermediate layer according to the present invention, when an amount of the coarse powder to be used in the coarse powder slurry is A g, and an amount of the fine powder is B g, the ratio of the amount of the coarse powder and that of the fine powder is preferably 0.1xe2x89xa6B/(A+B)xe2x89xa60.5. The reason is that if B/(A+B) less than 0.1, an amount of the fine powder is little so that a tight film is hardly obtained, while if B/(A+B) greater than 0.5, an amount of the fine powder is too much so that sintering property is too high whereby sintering crack and peeling of the film are likely caused.
A substance to be doped to lanthanum chromite according to the present invention is not particularly limited. There may be employed Ca, Sr, Mg, Co, Zn, Ti, Li, Cu, etc. In the viewpoint that a tight film can be formed at the sintering temperature of the interconnector film of 1300 to 1550xc2x0 C., lanthanum chromite containing Ca is preferred.
The synthesizing method of the lanthanum chromite powder according to the present invention is not particularly limited. There may employed a coprecipitation method, powder mixing method, spray thermal decomposition method, sol gel method, evaporation to dryness method, etc. Of these, the spray thermal decomposition method is preferred in the viewpoints that uniformity of the composition is high, migration of impurities such as silica, iron, etc. is little, operation step is short (cost is cheap), etc.
In the present invention, a solution of a slurry for preparing a ceramics intermediate layer and a tight ceramics film may contain the following.
Binder: PVA, EC (ethyl cellulose), etc. An amount thereof is preferably 0.1 to 10 parts by weight to 10 parts by weight of the solvent.
Difficultly volatile solvent: xcex1-terpineol, etc. An amount thereof is preferably 10 to 80 parts by weight to 100 parts by weight of the solvent.
Solvent: ethanol, 2-propanol, methanol, etc. An amount thereof is preferably 20 to 90 parts by weight to 100 parts by weight of the solvent.
Dispersant: polyoxyethlene alkylphosphate, CTAB, etc. An amount thereof is preferably 0.1 to 4 parts by weight to 100 parts by weight of the solvent.
Anti-foaming agent: sorbitan sesquioleate, etc. An amount thereof is preferably 0.1 to 4 parts by weight to 100 parts by weight of the solvent.
As a structure of an interconnector for SOFC according to the present invention, it is preferred to form a lanthanum chromite film at the oxidative atmosphere side and a nickel oxide film at the reductive atmosphere side. The reason is that in the interconnector comprising only lanthanum chromite, conductivity is markedly lowered at the reductive atmosphere side, and when a nickel oxide film is formed at the reductive atmosphere side, nickel oxide is changed to nickel to improve conductivity.
In the present invention, a sintering temperature of the interconnector film is preferably 1300 to 1550xc2x0 C. The reason is that if it is less than 1300xc2x0 C., an activity of a sintering aid (in Ca doped, calcium chromate and in Sr doped, strontium chromate) for lanthanum chromite is low and a tight film can hardly be obtained, while if it exceeds 1550xc2x0 C., in the formation of a solid electrolyte type fuel cell, sintering shall be carried out at higher temperature than the other material and thus, it is not practical. Incidentally, an atmosphere at the sintering is preferably air atmosphere in view of the other material (particularly an air electrode material).
An average gain size of nickel oxide powder in the present invention is preferably 0.1 to 20 xcexcm. The reason is that if it is less than 0.1 xcexcm, sintering property is too high so that sintering crack, peeling of the film, etc. may be caused, while if it exceeds 20 xcexcm, sintering property is too low so that it is difficult to form a tight film.